Understanding how seed black holes (BHs) grow into intermediate‐mass and supermassive black holes (IMBHs and SMBHs, respectively) has important implications for the duty cycle of active galactic nuclei (AGN), galaxy evolution and gravitational wave astronomy. Most studies of the cosmological growth and merger history of BHs have used semianalytic models and have concentrated on SMBH growth in luminous galaxies. We have developed a ‘hybrid method’ that combines high‐resolution cosmological N‐body simulations for the haloes' merger history, with semi‐analytical recipes for BH pair dynamics and BH gas accretion. We track the assembly of BHs over a large range of final masses – from seed BHs to SMBHs – over widely varying dynamical histories. We used the dynamics of dark matter haloes to track the evolution of seed BHs in three different gas accretion scenarios. We have found that growth of a Sagittarius A*– size of SMBH reaches its maximum mass MSMBH=∼106 M⊙ at z∼ 6 through early gaseous accretion episodes, after which it stays at near constant mass. At the same redshift, the duty cycle of the host AGN ends, hence redshift z= 6 marks the transition from an AGN to a starburst galaxy which eventually becomes the Milky Way. By tracking BH growth as a function of time and mass, we estimate that the IMBH merger rate reaches a maximum of Rmax= 55 yr−1 at z= 11. From IMBH merger rates we calculate NULX= 7 per Milky Way type galaxy per redshift in redshift range 2 ≲z≲ 6.

We explore the growth of ≤107 M⊙ black holes that reside at the centres of spiral and field dwarf galaxies in a Local Group type of environment. We use merger trees from a cosmological N‐body simulation known as Via Lactea 2 (VL‐2) as a framework to test two merger‐driven semi‐analytic recipes for black hole growth that include dynamical friction, tidal stripping and gravitational wave recoil in over 20 000 merger tree realizations. First, we apply a Fundamental Plane limited (FPL) model to the growth of Sgr A*, which drives the central black hole to a maximum mass limited by the black hole Fundamental Plane after every merger. Next, we present a new model that allows for low‐level prolonged gas accretion (PGA) during the merger. We find that both models can generate an Sgr A* mass black hole. We predict a population of massive black holes in local field dwarf galaxies – if the VL‐2 simulation is representative of the growth of the Local Group, we predict up to 35 massive black holes (≤106 M⊙) in Local Group field dwarfs. We also predict that hundreds of ≤105 M⊙ black holes fail to merge, and instead populate the Milky Way halo, with the most massive of them at roughly the virial radius. In addition, we find that there may be hundreds of massive black holes ejected from their hosts into the nearby intergalactic medium due to gravitational wave recoil. We discuss how the black hole population in the Local Group field dwarfs may help to constrain the growth mechanism for Sgr A*.

We present an isolated Milky Way-like simulation in GADGET2 Nbody SPH code. The Galactic disk star formation rate (SFR) surface densities and stellar mass indicative of Solar neighbourhood are used as thresholds to model the distribution of stellar mass in life friendly environments. SFR and stellar component density are calculated averaging the GADGET2 particle properties on a 2D grid mapped on the Galactic plane. The peak values for possibly habitable stellar mass surface density move from 10 to 15 kpc cylindrical galactocentric distance in 10 Gyr simulated time span. At 10 Gyr the simulation results imply the following. Stellar particles which have spent almost all of their life time in habitable friendly conditions reside typically at ∼ 16 kpc from Galactic centre and are ∼ 3 Gyr old. Stellar particles that have spent ≥ 90% of their 4 − 5 Gyr long life time in habitable friendly conditions, are also predominantly found in the outskirts of the Galactic disk. Less then 1% of these particles can be found at a typical Solar system galactocentric distance of 8 − 10 kpc. Our results imply that the evolution of an isolated spiral galaxy is likely to result in galactic civilizations emerging at the outskirts of the galactic disk around stellar hosts younger than the Sun.

Primordial stars are likely to be very massive (≥ 30  M⊙), form in isolation, and will likely leave black holes as remnants in the centres of their host dark matter haloes. We expect primordial stars to form in haloes in the mass range 106–1010  M⊙. Some of these early black holes, formed at redshifts z≳ 10, could be the seed black holes for a significant fraction of the supermassive black holes (SMBHs) found in galaxies in the local Universe. If the black hole descendants of the primordial stars exist, their merger with nearby SMBHs may be a prime candidate for long wavelength gravitational wave detectors. We simulate formation and evolution of dark matter haloes in a Λ cold dark matter universe. We seed high‐redshift dark matter haloes with early black holes, and explore the merger history of the host haloes and the implications of black hole kick velocities arising from their coalescence. The central concentration of low‐mass early black holes in present‐day galaxies is reduced if they experience even moderate kicks of tens of km  s−1. Even such modest kicks allow the black holes to leave their parent halo, which consequently leads to dynamical friction being less effective on the low‐mass black holes that were ejected, compared to those still embedded in their parent haloes. Therefore, merger rates with central SMBHs in the largest halo may be reduced by more than an order of magnitude. Using analytical and illustrative cosmological N‐body simulations, we quantify the role of kicks on the merger rates of black holes formed from massive metal‐free stars with SMBHs in present‐day galaxies.

We present a comparative study of recent works on merger time‐scales with dynamical friction and find a strong contrast between idealized/isolated mergers and mergers from a cosmological volume. Our study measures the duration of mergers in a cosmological N‐body simulation of dark matter, with emphasis on higher redshifts (z≤ 10) and a lower mass range. In our analysis we consider and compare two merger definitions: tidal disruption and coalescence. We find that the merger‐time formula proposed by Jiang et al. describes our results well and conclude that cosmologically motivated merger‐time formulae provide a more versatile and statistically robust approximation for practical applications such as semi‐analytic/hybrid models.